1. Both react with oxygen and water in the atmosphere to form acids which cause acid rain.
2. Both lead to the formation of PM10 (Particulate Matter 10 micrometers in diameter) in urban areas.
3. Both are released to the atmosphere in large quantities through combustion processes and are currently regulated by the EPA.
4. Sulfur oxides and nitrogen dioxide are respiratory irritants when present in large quantities
1. Sulfur oxides come from sulfur contaminants in fuels. However, most nitrogen oxides are formed by the reaction of atmospheric nitrogen with oxygen in high temperature flames. In essence, sulfur oxides are formed from things that we take from the ground and nitrogen oxides are formed from things that are in the air.
2. The formation of nitrogen oxides in a flame can be controlled by altering temperature, oxygen content, or time. However, the same can't be done for sulfur oxides.
3. The ultimate product of sulfur oxides removed in control equipment is CaSO4. CaSO4 is a low-solubility solid that can be placed in landfills. We don't have as much luck with nitrogen oxides. The final products of nitrogen oxide cleanup are N2 and O2 which will be returned to the atmosphere.
One of the major advantages to this technique is that it is cheap. The disadvantages are that it requires a larger firebox without a higher combustion rate. Also, it is difficult to get complete burning of the fuel in the second stage. Therefore, the amount of unburned fuel and/or carbon monoxide in the exhaust gas increases.
Many of these processes require the addition of a reducing agent to the combustion gas stream to take oxygen away from NO. In automobile engines, a platinum-rhodium catalyst is used. The reaction is:
On the other hand, for power plants and other large furnaces, there are many choices of reducing agents. However, the most popular is ammonia. The desired conversion reaction is:
However, there is always some oxygen present. This oxygen causes reactions like the following:
If the above reaction occurs, the NO2 is reduced by the following reaction:
All of these reactions are expensive to carry out. They can occur either over a zeolite catalyst or in a gas stream in a part of a furnace where the temperature is between 1600 and 1800 degrees Fahrenheit. If the temperature is greater than 1800 degrees, the NO content increases rather than decreases, which exactly what we DON'T want. The dominant reaction is: